Photoinduced reactions of functionalized monomers, oligomers, and polymers play a prominent role in technologies that contribute an estimated $25 billion per year to the world economy. Important commercial applications include the ultraviolet curing of coatings, the photoimaging of semiconductor chips, and the light-driven storage and output of visual information.
Photochemical or photoinitiated reactions occur when a reactive species is produced on exposure of the reaction mixture to light. The simplest mechanism for processes of this type involves the direct photochemical conversion of a substrate to a final product (eq. 1). If the substrate does not absorb the incident radiation, or does not form a reactive intermediate on exposure to the radiation, a second compound, referred to as ##STR1## a photoinitiator (P), can be added that absorbs incident light strongly and undergoes a photochemical transformation to one or more reactive species I (eq. 2). Interaction of I with the substrate results in product formation (eq. 3). Since the photoinitiator and substrate serve different functions, it is possible to optimize the properties of one without affecting the desirable features of the other. This inherent flexibility of a two-component system greatly simplifies the task of designing radiation-sensitive materials.
The species (I) can function as a true catalyst of reaction and suffer no permanent change in composition. Alternatively, (I) can be consumed while initiating a chain reaction of the substrate. Since, in either case, the reactive species produced by the action of a single photon may result in the conversion of several substrate molecules to product, the effective quantum efficiency (number of product forming events per photon absorbed) can exceed unity. This multiplicative response constitutes chemical amplification of the initial photochemical act and affords a means of designing materials with high radiation sensitivity.
The majority of commercially important photoinitiators are nonmetallic compounds that generate radicals and/or strong acids upon irradiation. Well-studied examples include benzoin and benzoin ethers, benzyl ketals, benzophenones plus hydrogen atom donors, thiol-ene systems, and onium salts belonging to the aryldiazonium, triarylsulfonium, and diaryliodium families. Of the relatively few transition metal-containing photoinitiators reported to date, most are organometallic complexes possessing photolabile ligands such as carbon monoxide, olefins, and carbocyclic rings. While the details of the mechanisms of initiation in these systems are sketchy, the photoinduced formation of a coordinatively unsaturated metal center appears to be a central feature.
The ability of classical metal ammine complexes to function as photoinitiators has been reported by Kutal, et al. In the Journal of the Electrochemical Society, Vol 134(9), 2280, 1987, Kutal and Willson reported that films spin-coated from solutions containing the copolymer of glycidyl methacrylate and ethyl methacrylate along with the transition metal coordination complex, Co(NH.sub.3).sub.5 Br! (ClO.sub.4).sub.2 undergo crosslinking upon irradiation at 254 nm and subsequent heating at 70.degree. C. The mechanism of crosslinking was determined to proceed in two distinct stages: (i) the primary photochemical process involving redox decomposition of the cobalt complex; and (ii) one or more thermally activated reactions between the decomposition products and the pendant epoxide groups on the copolymer. The reactive species responsible for the photoinduced crosslinking by Co(NH.sub.3).sub.5 Br! (ClO.sub.4).sub.2 was not elucidated in this work, but it was hypothesized. to be either a released ammonia molecule (neutral base catalysis) or cationic cobalt (II) complex (cationic catalysis).
In the Journal of Coatings Technology, July 1990, Kutal, Weit, MacDonald and Willson reported that Co(NH.sub.2 R).sub.5 X.sup.n+ complexes, where R is methyl or n-propyl and X is Cl.sup.- or Br.sup.-, photoinitiate crosslinking reactions in films of the copolymer of glycidyl methacrylate and ethyl acrylate at 254 nm. Irradiation of the cobalt complex at this wavelength causes efficient photoredox decomposition of the complex from a ligand-to-metal charge transfer excited state with release of several equivalents of free alkylamine. Even in the presence of oxygen, the decomposition quantum yields in solution can exceed 40%. The quantum yields for the alkylamine cobalt complexes are uniformly higher than those reported for the comparable ammonia complexes. It was also observed that Co(NH.sub.2 Me).sub.5 X.sup.2+ exhibits a greater photosensitivity than Co(NH.sub.3).sub.5 X.sup.2+ in the crosslinking reaction, suggesting that the initiating species is the substituted amine or ammonia (neutral base catalysis), and that the sensitivity is a function of the basicity of the amine. See also Advances in Resist Technology and Processing VIII, Volume 1466 (1991).
U.S. Pat. No. 3,794,497 to MacDonald, et al. discloses a means for recording an image which includes subjecting to light a layer that includes a vitamin B.sub.12 derivative and a polymerizable monomer. The invention appears to be based on the knowledge that vitamin B.sub.12, which contains an alkyl-cobalt bond, releases an alkyl radical on exposure to light (see column 2, lines 53-63). The patent also describes models of vitamin B.sub.12 which apparently do the same, or release an aryl radical (see column 3 and column 4, lines 1-13). The inventive aspect of the '497 patent appears to be the recognition that these types of vitamin B.sub.12 derivatives could be used as free radical initiators in image-recording processes.
Aslam, Polymer Photochemistry 5 41-48 (1984), reports that diacidobis(ethylenediamine)cobalt (III) complexes can cause the photopolymerization of vinyl monomers. Aslam states on page 42 of the article that irradiation results in the formation of radicals from the ligand, and therefore, polymerization proceeds by a radical mechanism, not an anionic mechanism. This point is underscored by Aslam's need to work in deaereated solution lit since, as noted above, oxygen inhibits radical reactions. Moreover, the polymerization was conducted in acidic solution, a medium that inhibits anionic polymerization.
Natarajan and Santappa, "Polymerization of Acrylamide and Methacrylamide Photoinitiated by Azidopentamminecobalt(III) Chloride", Journal of Polymer Science, Part A-1, Vol 6, 3245-3257 (1968), describe the kinetics of polymerization of acrylamide and methacrylamide photoinitiated by azidopentamminecobalt(III) chloride in aqueous acidic media. Under the experimental conditions employed, no photoaquation (i.e. release of azide anion) was observed, yet photopolymerization still occurs. Addition of azide anion had a negligible effect upon the rate of photopolymerization. The chloride ion, the anion that would be photogenerated in the system investigated by Aslam, is present as the counter-ion in the Natarajan complex and had no effect on the rate of polymerization. The authors state that initiation is photochemical in nature.
Conspicuously absent from the current catalogue of photoinitiators are those that undergo photochemical release of an anionic initiating species. Such an initiator would be of great value to induce light-catalyzed polymerization or crosslinking of a wide range of monomers, oligomers and polymers. For example, aldehydes and ketones, as well as certain ethylenically unsaturated monomers, undergo anionic polymerization or crosslinking, including ethylene, 1,3-dienes, styrene and .alpha.-methyl styrene, acrylates and methacrylates, acrylonitrile, methacrylonitrile, acrylamide and methacrylamide. Certain monomers also undergo anionic ring-opening polymerization or crosslinking reactions, including N-carboxy-.alpha.-amino anhydrides, cyclic amides, cyclic esters, epoxides and siloxanes.
Since anion-initiated polymerization is generally less sensitive to inhibition by oxygen (O.sub.2) than is radical initiated polymerization, it is a preferred process for many applications conducted in the ambient atmosphere. One of the goals of this invention is to develop polymerization methods that do not involve free radicals as polymerization initiating species.
In light of this, it is an object of the present invention to provide photoinitiators for anionic polymerization or crosslinking.
It is another object of the present invention to provide a method to select compounds for use as anionic photoinitiators.